Shoes such as walking, running, tennis, basketball, aerobic and soccer shoes must distribute the force of shoe-support surface impact and fit tightly on the user's foot. The force distribution and tightness of the fit needs to be greatest when the largest forces are being applied between the shoe and the support surface. It has been typical to rely on the sole to distribute the force and to tighten the shoe as much as possible, and physically bearable, to prevent or at least minimize relative movement of the foot in the shoe at times when maximum forces are transmitted between the support surface and the shoe. As a practical matter, the shock absorption is usually inadequate and such a fit is excessively tight during most times and quite frequently is uncomfortable, can lead to numbness and, in extreme cases, can even result in injuries. Thus, a compromise is frequently reached in the design and composition of the sole and by tightening the shoe on the foot more than is necessary for the small forces that are applied and less than is desired to prevent relative movement of the foot in the shoe when large forces are applied. Consequently, the shock absorbtion characteristics and the fit of such shoes is almost always other than what it should be.
Up to now, little or no consideration has been given to the shock absorbtion characteristics and the relative tightness of street and sport shoes, particularly light-weight, highly mobile shoes such as running, tennis, track and field and contact sport shoes. The lightness of such shoes and the lack of an adequate analysis of the interaction between the shoe, the user's foot and the support surface has led to the practice of relying on the elastomer sole to distribute the impact force and simply tightening the shoe to suit the user's taste, feel or preference. In some instances, the shoe might be too loose and not infrequently, slipped significantly relative to the foot in a particularly strenuous maneuver such as a sudden change in direction when turning. In addition, the impact force to the foot has not been adequately distributed.
Upon closer analysis, however, it becomes apparent that there are distinct phases in the use of a shoe, particularly a sport shoe, when forces applied by the foot to the shoe momentarily greatly exceed the normally encountered forces. During heel strike, for example, there are forces generated by both the player's weight which tend to concentrate the impact force in the area of the heel and there is the deceleration of the foot which tends to move the foot in a forward direction relative to the shoe. Such movements may be relatively small, say in the order of no more than a few millimeters but they are present and, typically, they are repeated thousands of times during play. This force distribution and foot slippage can lead to discomfort, skin irritation from rubbing between the foot and the shoe, injury and energy losses, which though small, are highly undesirable, particularly in competitive sports.